Aircraft have various safety-critical components which must be driven by an actuator between a first position and a second position. Examples of such components are a landing gear leg which is driven by the actuator between a raised and a lowered position relative to the fuselage; or flight control devices such as ailerons, spoilers or rudders.
Most safety-critical components on an aircraft, including those mentioned above, are designed so that net external load from aerodynamic forces and mass forces acting on the aircraft component tends to drive it towards one of the positions (for instance the extended position in the case of a landing gear component, or the neutral position in the case of a flight control device).
The actuator for such safety-critical components should of course be of high integrity and should have a very low risk of failure. For example, it is important for there to be provided a secondary, or emergency, extension system for extending the landing gear leg from the up-locked position (before landing) and for opening the landing-gear bay doors, in the event of a loss of the normal extension system. Such an emergency extension system should be required not only to extend the landing gear from its up-locked position, but also from any intermediate position in the event of a failure of the gear to retract fully for example.
There is a desire to reduce the reliance on hydraulic systems in large commercial aircraft and thus there is currently a desire to use electric actuators, where previously hydro-mechanical actuators were used.
One type of electric actuator which is commonly used has a threaded screw and a threaded nut which is driven by an electric motor to impart relative linear motion between the screw and the nut. However conventional perception is that there is a high risk of failure in the interface between the screw and the nut, so where such an actuator is used to drive a safety-critical component then the system must be designed to be failure tolerant. For instance in WO 2007/099333 the actuator is a series redundant actuator consisting of two back-to-back roller screws each driven by an independent electric motor. WO 2008/057106 describes an actuator with a threaded output screw which is driven by three roller nuts, each nut being driven by a different motor module. The problem with such conventional actuators is that although they have a high degree of failure tolerance, they are also large and bulky.